Sigma-delta modulators (SDMs) are devices that may be used in analog-to-digital converters. SDMs may also be referred to as delta-sigma modulators (DSMs). SDMs may use pulse-density modulation to convert an analog input signal to a digital pulse train whose spacing contains information representative of the analog input signal. SDMs are described at, e.g., U.S. Pat. No. 7,279,990 by Hasegawa, which is hereby incorporated by reference herein in its entirety. Sigma-delta modulation may be used in phase-locked loops (PLLs), e.g., for generating a modulation signal that modulates a frequency division ratio of a comparator or frequency divider of a PLL circuit. A variety of SDM structures are known. FIG. 1 shows a known SDM circuit 100 that is referred to as a feed-forward SDM structure. An analog input signal 110 is provided to a subtractor 120 that provides an output 125 to a loop filter 130, which may be an integrator. An output 135 of the loop filter 130 is added to the analog input signal 110, and the resulting output 145 is quantized by an analog-to-digital converter (ADC) 150. The resulting digital signal 190 is fed back via a digital-to-analog converter (DAC) 160 to the subtractor 120.
Noise leakage is a challenge facing circuit designers employing SDM techniques. Quantization noise associated with ADC 150 and error associated with DAC 160 are factors that degrade overall system performance. It is desirable to address such errors and noise efficiently.